Zinc/air cells are typically in the form of a miniature button cells which have particular utility as batteries for electronic devices and hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 16 mm, preferably between about 4 and 12 mm and a height between about 2 and 9 mm, preferably between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes. Zinc air cells can also be in other shapes, for example, elliptical or shapes having at least one flat side such as, prismatic, or rectangular shape.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each typically have a cup shaped body with integral closed end and opposing open end. After the necessary materials are inserted into the anode and cathode casings, the open end of the cathode casing is typically inserted over the open end of the anode casing with insulating material therebetween and the cell sealed by crimping. The anode casing can be filled with a mixture comprising particulate zinc. Typically, the zinc mixture contains mercury and a gelling agent, and the zinc mixture becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The cathode casing contains an air diffuser (air filter) which lines the inside surface of the cathode casing's closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent air holes in the surface of the closed end of the cathode casing. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the exposed side of the air diffuser not contacting the air holes. An ion permeable separator is typically applied over the catalytic material so that it faces the open end of the cathode casing.
The cathode casing can typically be of nickel plated cold rolled steel or nickel plated stainless steel, for example, with the nickel plate forming the cathode casing's outside surface and cold rolled steed or stainless steel forming the casing's internal layer. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. In such embodiment the nickel layer typically forms the anode casing's outside surface and the copper layer forms the anode casing's inside surface. The copper inside layer is desirable in that it provides a highly conductive pathway between the zinc particles and the cell's negative terminal at the closed end of the anode casing. An insulator ring of a durable, polymeric material can be inserted over the outside surface of the anode casing. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed.
After the anode casing is filled with the zinc mixture and after the air diffuser, catalyst, and ion permeable separator is placed into the cathode casing, the open end of the cathode casing can be inserted over the open end of the anode casing with the insulator ring therebetween. The peripheral edge of the cathode casing can then be crimped over the peripheral edge of the anode casing to form a tightly sealed cell. The insulator ring around the anode casing prevents electrical contact between the anode and cathode cups. A removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell. A portion of the closed end of the anode casing can function as the cell's negative terminal and a portion of the closed end of the cathode casing can function as the cell's positive terminal.
Typically, mercury is added in amount of at least one percent by weight, for example, about 3 percent by weight of the zinc in the anode mix. The mercury is added to the anode mix to improve interparticle contact between zinc particles in the anode mixture. This in turn improves electrical conductivity within the anode and thus results in increased cell performance, for example, higher actual specific capacity of the zinc (Amp-hour/g). Also addition of mercury tends to reduce the hydrogen gassing which can occur in the zinc/air cell during discharge and when the cell is placed in storage before or after discharge. The gassing, if excessive, increases the chance of electrolyte leakage, which will reduce cell performance and can damage or destroy the hearing aid or other electronic component being powered.
U.S. Pat. No. 3,897,265 discloses a representative zinc/air button cell construction with an anode casing inserted into the cathode casing. There is disclosed an insulator between the anode and cathode casings. The anode comprises zinc amalgamated with mercury. The cell includes an assembly comprising an air diffuser, cathode catalyst, and separator at the closed end of the cathode casing facing air holes in the surface of the cathode casing.
U.S. Pat. No. 5,279,905 discloses a miniature zinc/air cell wherein little or no mercury has been added to the anode mix. Instead, the inner layer of the anode casing has been coated with a layer of indium. The disclosed anode casing can be a triclad material composed of stainless steel plated on the outside surface with nickel and on the inside surface with copper. The copper layer is at least 1 microinch (25.4×10−6 mm). The reference discloses coating the copper layer on the anode casing's inside surface with a layer of indium. The indium layer is disclosed as being between about 1 microinch and 5 microinches (25.4×10−6 mm and 127×10−6 mm).
U.S. Pat. No. 6,447,947 B1 discloses a miniature zinc/air cell wherein little or no mercury has been added to the anode mix. The disclosed anode casing can be a triclad material composed of stainless steel plated on the outside surface with nickel and on the inside surface with copper. The anode can is heat treated with an inert gas at elevated temperatures before anode material comprising a zinc slurry is inserted into the casing. The heat treating process of the anode can treats the copper surface texture and removes oxides therefrom. This significantly reduces gassing during cell discharge and storage and eliminates the need to add mercury to the anode material.
It has been determined that an occasional problem can occur during and after insertion of the zinc-air battery into certain hearing aid battery compartments which can result in diminished electrical contact between the battery and hearing aid. This in turn can lead to scratching background noises or intermittent loss in hearing aid output tone and amplification. This problem can occur regardless of whether there is mercury added to the anode, for example, 3 percent by weight or less mercury based on zinc or even if the anode contains zero added mercury.
The problem has been traced to inadequate electrical contact which may occasionally occur between the cathode casing (cathode cup) and certain electrical contacts within the hearing battery compartment, particularly when the contact terminal within the battery compartment is of stainless steel.
It is thus desired to improve the electrical contact between the zinc-air cell cathode cup and its contact within the hearing aid or other device being powered. It is desired to produce such contact so that the hearing aid or other audio device being powered will produce clear output signals without distracting scratching background noises or intermittent loss in output.